The instant invention is directed to golf balls, and more particularly to a ball having the optimal core compression, core diameter, cover hardness, and dimple configuration to provide superior playability characteristics with respect to softness and spin without sacrificing superior distance capabilities.
There are a number of physical properties that affect the performance of a golf ball. The core of the golf ball is the source of the ball""s energy. Among other things, the core affects the ball""s xe2x80x9cfeelxe2x80x9d and its initial velocity. The xe2x80x9cfeelxe2x80x9d is the overall sensation transmitted to the golfer through the golf ball after striking a ball. The initial velocity is the velocity at which the golf ball travels when first struck by the golf club. The initial conditions, which includes velocity, together with the ball""s trajectory, determine how far a shot will travel.
Until the late 1960""s most golf balls were constructed as three-piece wound balls. In the three-piece wound ball, a solid or liquid-filled center is wound with rubber windings to form a core, which is then surrounded with a cover of compounds based on natural (balata or gutta percha) or synthetic transpolyisoprene. During the manufacturing process, after the liquid-filled center is formed, it is frozen to make it as hard as possible so that it will retain its spherical shape while the rubber thread is wrapped around it.
These three-piece wound balls were known and are still known to provide acceptable flight distance and soft feel. Additionally, due to the relative softness of the balata cover, skilled golfers are more able to impart various spins on the ball in order to control the ball""s flight path (e.g. xe2x80x9cfadexe2x80x9d or xe2x80x9cdrawxe2x80x9d) and check characteristics upon landing on a green.
With the advent of new materials developed through advances and experimentation in polymer chemistry, two-piece golf balls were developed. The primary difference between a two-piece golf ball and a three-piece golf ball is the elimination of the rubber thread windings found in the three-piece balls. A relatively large solid core in a two-piece ball takes the place of the relatively small center and thread windings of a three-piece ball core having the same overall diameter. With the elimination of the thread windings, there is no need to freeze the core during the manufacturing process of the two-piece golf ball.
Two-piece balls have proven to be more durable than three-piece balls when repeatedly struck with golf clubs and more durable when exposed to a variety of environmental conditions. An example of these environmental conditions is the high temperature commonly experienced in an automobile trunk. In addition, two piece balls are typically less expensive to manufacture than the three-piece wound balls. However, two-piece balls are, in general, considered to have inferior characteristics of feel and workability when compared to three-piece balls. Generally and historically, two piece balls use harder cover materials for increased durability. The xe2x80x9chardnessxe2x80x9d of a golf ball can affect the xe2x80x9cfeelxe2x80x9d of a ball and the sound or xe2x80x9cclickxe2x80x9d produced at contact. xe2x80x9cFeelxe2x80x9d is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball""s diameter. Generally, the lower the compression value, the softer the xe2x80x9cfeel.xe2x80x9d Consequently, two-piece golf balls have a higher initial velocity. In addition, typically two-piece golf balls have more potential energy, which is derived primarily from the core. The cores in two piece golf balls are typically larger than the centers in three-piece golf balls.
In contrast, three-piece golf balls with their smaller centers historically use softer cover materials. These softer cover materials result in a lower initial velocity when compared to two-piece golf balls. However, this difference in the initial velocity may be somewhat made up by the windings in the three-piece golf ball.
In addition to manipulating the core and cover of a golf ball, for many years golf balls have been made with surface indentations or depressions, called dimples, to improve their aerodynamic properties in flight. Specifically, ball manufacturers have looked to dimple configurations in an effort to design a ball with superior distance capabilities. Many efforts have been made to select the optimum number, size and shape of dimples as well as their disposition around the outer surface of a generally spherically shaped golf ball.
Ball manufacturers are bound by regulations of the United States Golf Association (USGA) which control many characteristics of the ball, including the size and weight of the ball, the initial velocity of the ball when tested under specified conditions, the overall distance the ball travels when hit under specified test conditions, and the ball""s aerodynamic symmetry. Under USGA regulations, the diameter of the ball cannot be less than 1.680 inches, the weight of the ball cannot be greater than 1.620 ounces avoirdupois, the initial velocity of the ball cannot be greater than 250 feet per second when tested under specified conditions (with a maximum tolerance of +2%), the driver distance cannot exceed 280 yards when tested under specified conditions (with a test tolerance of +6%), and the ball must perform the same aerodynamically regardless of orientation.
While the USGA sets a limit for the distance a ball can travel under set test conditions, there is no upper limit on how far a player can hit a ball. For example, U.S. Pat. No. 5,575,477 discloses the projection of an icosahedron onto the ball as a basis for a dimple configuration. The balls twenty triangles are then further divided by six great circle paths which are devoid of dimples. U.S. Pat. No. 5,249,804 discloses a golf ball having a dimple pattern based on an icosahedron pattern consisting of twenty triangles. The preferred embodiment contains 362 dimples of two different diameters.
A problem with the prior art dimple configurations is that they fail to take into account other features of the ball, such as core size, core compression and cover hardness, which also influence how far a ball will travel.
U.S. Pat. No. 5,368,304 to Sullivan discloses a ball having a low spin rate, which in turn enables the ball to travel greater distances. According to the Sullivan patent, the low spin rate is the result of a soft core and hard cover. While the ""304 patent discloses the use of a soft core and hard cover to lower the spin rate, it does not disclose a dimple configuration for the ball.
Accordingly, it is an object of the instant invention to provide a two-piece golf ball that has a soft feel in combination with superior distance capabilities.
It is another object of the instant invention to optimize the combination of core compression, core size, core composition, dimple configuration, cover composition, and cover hardness to provide a two-piece golf ball, which travels great distances, and at the same time complies with USGA regulations.
It is yet another object of the instant invention to provide a two-piece golf ball having a synthetic cover material that achieves the sound, feel, playability and flight performance qualities of balata covered golf balls.
It is a further object of the instant invention to lower the cost of manufacturing a two-piece golf ball that has a soft feel in combination with superior distance capabilities.
It is still a further object of the instant invention is to provide a two-piece golf ball having superior distance, trajectory and flight stability.
Another object of the instant invention is to provide a two-piece golf ball having a surface divided into a plurality of polygonal configurations or shapes for the location of dimples for enhancing the aerodynamic properties of the golf ball.
The invention achieves the above-described objectives by providing a two-piece golf ball having a solid rubber core, a synthetic ionomer resin cover, and a xe2x80x9cicosahedronxe2x80x9d dimple pattern. The ball of the instant invention has a core compression in the range of 68 PGA to 78 PGA; a core diameter in the range of about 1.535 inches to about 1.545 inches; a cover hardness in the range of about 53 Shore D to about 59 Shore D, and a dimple pattern based on the geometry of an icosahedron. This combination has been found to produce a ball with superior distance capabilities, which also satisfies USGA regulations. The use of these properties in the golf ball of the instant invention is based on the recognition that it is the combination of the core compression, core composition, core size, cover composition, cover hardness, dimple configuration, dimple size and dimple shape that will produce a ball that will travel the greatest distance without compromising shot-making feel.
The cover material can be constructed from any relatively stiff material, for example, synthetic thermoplastic materials. Most notably these synthetic thermoplastic materials are ionomeric resins. lonomeric resins are polymers containing interchain ionic bonding. As is well known in the chemical arts, ionomeric resins are generally ionic copolymers of an olefin having from about two to about eight carbon atoms, such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar polymer matrix. Metal ions, such as sodium, lithium, zinc or magnesium are used to neutralize some portion of the acidic groups in the copolymer. This results in a thermoplastic elastomer, which exhibits enhanced flight characteristics and durability when compared to golf balls constructed with balata covers. However, the advantages gained by enhanced durability have been offset by the decreased playability properties.
The ionomers used in the cover composition are sold by E.I. Dupont De Nemours and Company under the name SURLYN. In an attempt to overcome the negative factors of the hard ionomer covers, DuPont introduced low modulus SURLYN ionomers in the early 1980""s. These SURLYN ionomers have a flexural modulus of from about 3000 to about 7000 PSI and hardness of from 25 to about 50 as measured on the Shore D scalexe2x80x94ASTM 2240. The low modulus ionomers are terpolymers, typically of ethylene, methacrylic acid and n- or iso-butylacrylate, neutralized with sodium, zinc, magnesium or lithium cations. E.I. DuPont De Nemours and Company has disclosed that the low modulus ionomers can be blended with other grades of previously commercialized ionomers of high flexural modulus from about 30,000 to 75,000 PSI to produce balata-like properties. However, soft blends, typically 52 Shore D and lower (balata-like hardness), are still prone to cut and shear damage.
The low modulus ionomers when used without high flexural modulus blends produce covers with very similar physical properties to those of balata, including poor cut and shear resistance. Worse, wound balls with these covers tend to go xe2x80x9cout-of-roundxe2x80x9d quicker than wound balls with balata covers. Blending with hard SURLYN ionomers was found to improve these properties.
It has now been discovered that a blend of very low modulus ionomers with an associated low acid level with an improved flow ionomer containing a medium acid level results in a golf ball cover with improved playability characteristics. For the purposes of the SURLYN ionomer resin grade designations, a low acid level is approximately 12% by weight, and a medium acid level is approximately 15% by weight.
As mentioned previously, in addition to manipulating the core and cover parameters in a golf ball, superior aerodynamic properties are also attributed to the dimple configuration on a golf ball. In the instant invention, the dimples are arranged on the surface of the golf ball based on the geometry of a icosahedron. This configuration is achieved by dividing the outer spherical surface of a golf ball by into a plurality of triangles for locating a plurality of dimples on the outer surface of the golf ball. This polygonal configurations is generally referred to herein as a xe2x80x9cicosahedronxe2x80x9d.
The outer surface has a plurality of dimples of different sizes. In one embodiment, the dimples are of first, and second sizes and are generally located to have a pattern associated with the triangles. Dimples are preferably circular in shape, but can have a non-circular shape within the scope of this invention.
The combination of the aforementioned core, cover and dimple specifications produces a golf ball that possesses noticeable improvements in playability (i.e. spin properties) without sacrificing the ball""s durability (i.e. impact resistance etc.) which in turn relates directly to the distance a ball will travel when struck. In addition, the instant invention provides a golf ball composition that exhibits the desired properties of the three-piece wound ball (e.g. long distance in combination with a soft feel), but with the lower manufacture cost associated with the two-piece ball. These and other objects of the instant invention will be apparent from a reading of the following detailed description of the instant invention.